Among conventional boots of this kind is a boot that includes a freely extendable and contractible bellows part, a large-diameter ring-shaped mounting part formed at one end in the axial direction of this bellows part, and a small-diameter ring-shaped mounting part formed at the other end in the axial direction, and is molded by blowing a material such as thermoplastic elastomer.
In the boot of this kind, the large-diameter mounting part is tightly fixed by one clamp on the outer peripheral surface of a shaft side end of one member of two members that can move relatively to each other and the small-diameter mounting part is tightly fixed by another clamp on the outer peripheral surface of a shaft of the other member.
With this, the bellows part can be flexibly deformed with the relative movement of the two members to prevent foreign matters from entering a movable mechanism part between the two members from outside and to prevent lubricating grease or the like filled in the movable mechanism part from leaking outside.
To be specific, there has been known a boot that is used for protecting a constant-velocity joint used for transmitting the driving force of an automobile. In the constant-velocity joint like this, the outside shape of the fixed part of a joint cup where the boot is fixed is not cylindrical but may be formed in the shape of a tripod that has three curved depressions. In this case, the inner periphery of the fixing part of the boot is formed in the shape of a tripod in accordance with the shape of the outer periphery of the fixing part of the cup and the outer periphery of the fixing part of the boot is formed in the shape of a cylinder. The outer periphery of the fixing part of the boot is fastened by a band, whereby the boot is fixed to the cup. At this time, three depressions formed in the outer periphery of the fixing part of the cup function as parts for preventing the boot from rotating with respect to the joint cup.
For example, Japanese Unexamined Utility Model Publication No. 1-118227 discloses a boot 121 in which a large-diameter ring part 122 is varied in thickness with respect to the cup formed in the shape of the above-described tripod in a state where the outer peripheral surface is kept in the shape of a ring and whose thick portions 123 are opposed to the depressions of the cup and whose thin portions 124 are opposed to the circumferential portions of the cup (see FIG. 6).
The boot 121 has two ribs 126 in the peripheral direction on the inner peripheral surface of the large-diameter ring part 122. A relief groove 127 is formed between the ribs 126 formed on the inner peripheral surface of each of the thick portions 123 (FIG. 7A). Further, a depression 128 formed between the ribs 126 in a connection portion 125 connecting the thin portion 124 and the thick portion 123 is formed in a shallower shape than the relief groove 127 in the thick portion 123 (FIG. 7B).